Digital circuits are used for storing and manipulating digital data. Data is stored in a binary form, and its manipulation is performed via logic operations. Transistors and diodes, the building blocks of digital circuits, function essentially as “switches” that either block or allow current flow. Operated at a bias limit, such switches may be in a state of either cutoff or saturation, corresponding respectively to either a state of practically no current or a state of maximal current. Circuits are designed to make use of this property, so that a transistor or diode serves as physical representation of a binary bit. Digital circuits are often comprised of smaller electronic circuits called “logic gates”. Each logic gate is an arrangement of such electrically controlled “switches”, and applies a function of Boolean logic on its input signals, i.e. currents and/or voltages. The output is an electrical current or voltage, being itself a representation of a single bit which can in turn control other logic gate(s).
There are several different types of logic gates:
Diode Logic (DL) gates use diodes to perform AND and OR logic functions. They are simple and inexpensive, but they tend to degrade digital signals rapidly, and cannot perform a NOT (inversion) function.
Resistor-Transistor Logic (RTL) gates use transistors operable to combine multiple input signals. These transistors also amplify and invert the resulting combined signal, so an additional transistor is often included to re-invert the output. This combination provides clean output signals and either inversion or non-inversion as needed. RTL gates are almost as simple as DL gates and remain inexpensive, but they draw significant amount of current from the power supply for each gate. Another limitation is that RTL gates cannot switch at high speeds used by modern computers.
Diode-Transistor Logic (DTL) gates use diodes to perform the logical AND or OR function, and then amplify the result with a transistor. These logic gates essentially take DL gates and add a transistor to the output, in order to provide logic inversion and to restore the signal.
An integrated circuit construction makes it more effective to replace the input diodes in DTL gates with transistors. The result is Transistor-Transistor Logic (TTL) gates, which became standard for a number of years. TTL devices use bipolar transistor switches and define the binary values as: 0-0.8V=‘0’, 2-5V=‘1’. They are inexpensive, but draw a lot of power (individual gates may draw 3-4 mA) and must be supplied with an input voltage of +5V. The low power Schottky versions of TTL chips draw 20% of the power, but are more expensive.
Emitter-Coupled Logic (ECL) gates are designed to operate at very high speeds, by avoiding the “lag” inherent when transistors are allowed to become saturated. The transistors in the logic gate are never completely cutoff or saturated, and remain within their active operating regions at all times. As a result, the transistors do not have a “charge storage” time, and can therefore change states much more rapidly. However, these gates demand substantial amounts of electrical current to operate correctly.
Complementary Metal Oxide Semiconductor (CMOS) devices are made from MOSFETS. They are much lower in power requirements than TTL devices and operate with a wide range of supply voltages (e.g. 3-18V), but are extremely sensitive to static electricity.
PMOS and NMOS (P- and N-channel Metal Oxide Semiconductor) devices offer higher component density than do TTL chips, but are, like CMOS, sensitive to damage from electrical discharge. This family does not have as many TTL chip equivalents as does the CMOS, and is used mainly for VLSI large scale integrated circuits.
WO 2006/077596, assigned to the assignee of the present application, discloses a device for implementing logic function using free electrons moving in vacuum. According to this technique, the device output is created by charging/discharging one or more floating electrodes in response to a certain input field, and the output is read as electric potential(s) on the charged/discharged floating electrode(s). The device includes one or more basic units of electrodes, configured to define vacuum space(s) for free charged particles' propagation, and includes an input assembly for supplying an input signal, and a floating electrode assembly accommodated proximal the input assembly and serving for reading an output signal therefrom. The floating electrode arrangement is configured to define at least one source of the free charged particles and at least one target toward which the charged particles are directed and is chargeable and dischargeable in response to the input signal thereby creating the output of the basic unit.